The field of art to which this invention pertains is aryl cyanate esters, i.e., cyanic acid esters of polyhydric phenols.
Industry is constantly searching for lighter, stronger and more resistant materials to be used in place of the materials used today. For example, the aerospace industry is devoting considerable effort to utilizing structural composites in place of metals. Structural composites based on thermoplastic or thermoset resins and glass or carbon fibers have been and are being used successfully in many parts of military and commercial aircraft. Thermoset resins which are being used in such applications are epoxy resins, bismaleimide resins, and cyanate ester resins.
Cyanate ester resins, which are finding increasing use in structural composites, adhesives and electrical grade insulation are based on the reaction products of polyhydric phenols and cyanogen halides. Such resins and their methods of preparation are described in U.S. Pat. Nos. 3,403,128 and 3,755,042. Additional patents which describe cyanate esters are U.S 3,987,230 and 4,330,658.
Commercially available cyanate esters are either crystalline solids having melting points above 70.degree. C. or are amorphous semisolids having viscosities at 25.degree. C. of more than 100,000 cps. Such cyanate ester resins are difficult to use in many commercial operations. Manufacturers continue to employ them in many processes because of their superior physical and electrical properties when cured. Such resins, however, are not suitable for fabrication of structural composites using wet filament winding, resin transfer molding and pultrusion processes. These newer processes are more economical than the labor intensive procedure of hand placing tacky, drapable prepregs onto a mold followed by vacuum bag construction and autoclave curing.
Wet filament winding, resin transfer molding and pultrusion all require rapid on-line wetting and impregnation of continuous fiber reinforcement. Such a procedure requires resin viscosities in the range of 100 to 10,000 cps together with a catalyzed pot-life of 2 to 24 hours and gelation within 0.5 to 10 minutes at temperatures of 150.degree.-180.degree. C. In order to obtain a suitable viscosity, crystalline and semisolid amorphous cyanate esters must be heated to elevated temperatures, which requires a compromise between the catalyzed pot-life requirement and the rapid gelation requirement.
Resin transfer molding processes are operable using resins having viscosities of less than 2000 cps with the preferred range being 100-200 cps. Pultrusion operations can be conducted with resins having viscosities at 400 to 5,000 cps with the preferred range being 400 to 1100 cps. The viscosity requirements for resins used in filament winding processes are 500 to 10,000 cps with the preferred range being 500 to 1500 cps.
There is a need then for cyanate ester resins which have low viscosities, long term stability and fast curing capabilities when properly catalyzed.